1. Field of the Invention
This invention relates to modulation of light beams and more specifically to an improved acoustooptic modulator.
2. Description of the Prior Art
Multi-channel laser beam systems are used, for instance, in laser writing applications, such as imaging patterns onto photo-resist using multiple laser beams for purposes of creating electronic circuit substrates. Such systems employ the well known acousto-optic modulator (AOM). In such a modulator, electrical energy is converted to acoustic waves by a piezoelectric transducer, and the acoustic waves modulate the incident laser (light) beams. The acoustic waves distort the optical index of refraction of the modulator body, which is typically crystalline material or glass, through which the laser beams pass. This distortion is periodic in space and time and thus provides a three dimensional dynamic diffraction grating that deflects or modulates the laser beams. Such acousto-optic devices are well known in broadband signal processing.
An example of such a modulator 10 is shown in FIG. 1A illustrating the exterior of the modulator body 14. The light beam 16 enters from the left surface of the body 14 and passes through the body 14. The horizontal lines are intended to suggest diffraction grating properties; it is to be understood that the molecules in the modulator body, compressed or stretched by the presence of acoustic waves, provide the effect of a three dimensional dynamic phase grating and this is not a conventional diffraction grating.
The electrical input signal ("input") is applied via input electrode 21 to the surface transducer electrode 20 on the modulator body 14. Electrode 20 is made of a thin platelet of piezoelectric material bonded to the surface of the modulator body 14. Electrode 20 also provides acoustic impedance matching. Light beam 16 enters the body 14 through a surface of body 14 orthogonal to the surface to which the piezoelectric electrode 20 is bonded. The frequency and power of this electrical input signal determines to what extent light beam 16 is deflected by passing through modulator body 14 due to the presence of the resulting acoustic wave. Conventionally an acoustic termination such as an acoustic absorber 22 is provided on the surface of the modulator body 14 opposite to the surface on which the electrode 20 is bonded and the electrical signal is applied. Alternatively, the surface of the modulator body opposite to the surface on which electrode 20 is bonded may be cut at an angle causing incident acoustic waves to reflect off-axis and eventually be absorbed by the modulator body.
Thus the electrical connection 21 with electrode 20 and ground electrode 24 is an electrical input port and the voltage (signal) applied thereto creates a spatially uniform electric field in the piezoelectric active regions of electrode 20 to cause the generation of a uniform acoustic wave traveling down the modulator body 14, which in turn, causes the intended deflection of the light beam 16. Due to photo-elastic coefficients of the modulator material 14, the actual effect is caused by appreciable variations in the refractive index of the modulator body 14 which in effect creates a moving (dynamic) diffraction grating traveling at the speed of sound with a grating strength determined by the input electrical power. The angle of deflection of the output light beam and its magnitude as produced by the moving diffraction grating depends on the frequency and the amplitude of the acoustic wave.
FIG. 1A shows only a single electrode 20 for modulating a single incident light beam 16. "Light beam" in this context refers to any electromagnetic radiation which may be so modulated, including not only visible light but also ultraviolet light and other frequencies including infra-red, etc., from a laser or other source.
In multi-channel laser beam systems, a plurality of laser (light) beams 16a, 16b, 16c, 16d are incident on a single modulator body (see FIG. 1B). The modulator body 14 has formed on its surface a corresponding number of electrodes 20a, 20b, 20c, 20d, there being one such electrode for each beam 16a, . . . , 16d to be modulated. Such a device has a plurality of electrodes 20a, 20b, 20c, 20d on the surface of the modulator body 14. Typically there are 4 or 8 or more such electrodes, each deflecting a corresponding incident beam. The physical size of each electrode can be very small for the case of a high speed modulator array, about a few hundred micrometers by a few millimeters each for modulator bandwidth on the order of tens of megahertz. It is a common practice to form such modulator electrode arrays using conventional photo-lithographic means to define the small electrodes. To provide electrical and acoustic isolation, the electrodes are made with a finite gap in between.
In laser imaging systems the intent is to form an array of tiny laser beam dots, modulated in time, on the imaging medium, the dots having a typical packing density of 300 to 300,000 or more dots per inch. Moving the modulated optical dot array in a direction nominally orthogonal to the dot array orientation, i.e. raster scanning, on an optically sensitive medium produces a recorded image of the modulating signal. Obviously, in order to print a continuous quality pattern, there should be no noticeable gap between adjacent laser beam dots on the optically sensitive medium.
Since the desired laser beam dots tends to be substantially smaller in diameter than the laser beams in the modulator array, optical imaging techniques are employed to reduce the laser beam diameters and to eliminate the gaps between adjacent modulated laser beams from a modulator array.
For adequate efficiency, acousto-optic modulators as described above are typically operated, in terms of power input, at watts or fractions of a watt RF power. RF power refers to the amount of applied electrical power at the input terminal. This power is partly converted into heat near the transducer region and causes pattern dependent thermal gradients in the interaction medium, which is the body of the acousto-optic modulator. Undesirably, these gradients may deflect the incident light beam, or beams, away from the optimal Bragg angle condition and cause the amount of light transmitted to change, depending on the recent modulation state history of the modulator. Pointing of the diffracted light rays coming from the modulator may also be adversely affected.